Hello all, a fellow engineering student and myself are designing an automated tennis ball cannon that is mounted onto a remote control robotic base. We have all kinds of goodies planned for the system, but I have some initial questions. I've done a good amount of research thus far too.

1. What is a good C:B ratio for pneumatic cannons? Right now, the prelim design uses a 3"x12" chamber, and a 2.5"x24" barrel. This is about a 0.75:1... we are not planning to make this super-powerful, mainly because school regulations/safety concerns, etc...

2. When using CO2, what are the typical storage containers used, and how many shots can you get out of them? (I'm sure this is dependent on the chamber size as well)

3. When using the 1" sprinkler valve, how fast does it open if you use it in the current setup? By this I mean, using the solenoid to actuate the valve via an electrical signal. Do you get the same performance as tapping in a blow gun?

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1. C:B ratio isn't as much of a factor in pneumatics as it is in combustions. A longer barrel will supply more power, as will a larger chamber. Not to mention that higher pressure yields better results. To answer the question, you should be fine with what you have, although if using regulated CO2 a smaller chamber would be a good idea to save gas.

2. Typical storage would be in a paintball tank or a larger 5-10 lb. tank for beverages. On a robot your best choice would be a 20 oz paintball tank with a regulator. I've never used them personally, but the Palmer's regulators seem to be the best.

3. I don't know the actual time in ms but I'm sure someone else will. Tapping in a blowgun will yield alot better performance, but electronics seems the only way to go if you are looking for a completely remote-controlled setup. One way to increase opening times but stay electronic is to get another smaller solenoid valve either 1/4" or 1/8" and tap that in instead of a blowgun. Check eBay, there are usually a couple cheap solenoid valves up for grabs there.

Good to see you've already done your research, sorry i don't know about valve opening times for the sprinkler valve.

Welcome to Spudfiles, your project is one I can't wait to see completed. Good luck!

1. Some CB tests have been done and they found that .8:1 works best for pneumatics.

2. If your using co2, I would use a 20 oz. paintball tank especially if it will be on the robot. You don't want a big 5 gallon tank (I don't think). This would need to be regulated and I would recommend getting a regulator at palmer-pursuit.com. I'm not sure how many shots you would get but I would guess around 20-40 with your chamber.

3. Using a solenoid isn't as fast as a blow gun but the performance isn't bad.

When it comes to C:B ratios, bigger is always better, but less efficient. If I remember my math right, the muzzle energy of your projectile should (theoretically, but not perfectly) increase with the LN of chamber volume, so its just a question of when you think increasing the volume is not worth it anymore. If fill time is not much of a factor I usually go with 4:1, but that's really overkill. Anything around 1:1 should be fine.

Yeah since you're using CO2, I would keep the c:b as low as possible. It really depends on what your going to be using the cannon for. If it's literally serving tennis balls to a player on the court, you'll be able to get by with a really small chamber as you'll you be using it to lob serves, relatively slow in cannon terms. If you have some other use for it, you may need more power and therefore a larger chamber and longer barrel.

Try a chamber at .5:1 with the 24" barrel and then play around with psi settings until you get the power you need. If you can't get the power you want that way, you'll need to up your chamber some. I can't see you needing to go beyond 1:1 for a tennis ball and a 24" barrel.

I would go with the Paintball CO2 tank, regulator set fairly low, and a relatively small chamber.

The big issue here is safety, a decent CO2 powered pneumatic can fire a tennis ball at deadly speeds. Heck, even the speed that a decent tennis player can get with their racket is unsafe at short range. So, for safety reasons (especially if the gun is controlled by the robot and not by a person) I would say the muzzle velocity has to be kept below 100 FPS. Even at 100 FPS a tennis ball may still be able to brake a window.

Since performance isn't an issue, stay with the solenoid activated valve. No need for anything more complex since you can't safely use the increased performance anyway.

A low C:B ratio may surely help conserving CO2.
How big is your robot going to be?
Maybe it could have its own on-board compressor

Note that you may improve the gun's efficiency further by using an even smaller chamber, but then mod the valve to make up for the loss of power.
When you are capable of building a robot, then you are surely also capable of having that robot press in a blowgun.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

Great information everyone!
I have considered the on-board compressor; however, we are going to have a significant number of electronics on this robot and some of the largest predicted expenses is the power source (batteries). If we can keep power draw down by using a CO2 bottle, then we'll go that method. Thanks for the suggestion though.

Sounds like we'll be OK just using the solenoid to actuate the valve, if we feel that its performance is not sufficient, we'll reevaluate and maybe add another method.

So to have the proper amount of CO2 enter the chamber, will we need just a simple regulated CO2 bottle set to say 100psi? Or will there need to a pressure switch plumbed into my system? Like this:
http://www.bcarms.com/product_info.php? ... a457048466I imagine we could use this switch to "ready" the system for launch.

The plan is to fire the cannon with the press of a button on a remote control (operated by a human). The chamber must auto-fill to the proper pressure, when the previous shot is expelled. Additionally, we need to be able to shoot more than one ball without manually reloading it (hence the computer control requirement portion of the design project).

The overall robot will be driven by two wheelchair motors (2wd), as well as the cannon will be mounted on a four degrees of freedom turret system (pan left/right, tilt up/down).

^^^That's a nice idea!
I searched for some pictures of this actually built, but all I could find were more gif and idea threads. Anyone have any pictures of the thing built? I believe we'll be able to implement the idea, possibly with an electric solenoid because we're going to have an additional power source (not just CO2).

Also, I read that the sprinkler valves have a "slow resetting time." What is considered "slow?" I mean, we are not trying to make this a rapid fire gun/cannon, just one that resets itself.